linux-kernel - Re: [PATCH] RAS: Add a tracepoint for reporting memory controller events

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Date:	Thu, 31 May 2012 21:37:39 +0200
From:	Borislav Petkov <bp@...64.org>
To:	Mauro Carvalho Chehab <mchehab@...hat.com>
Cc:	"Luck, Tony" <tony.luck@...el.com>,
	Linux Edac Mailing List <linux-edac@...r.kernel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Aristeu Rozanski <arozansk@...hat.com>,
	Doug Thompson <norsk5@...oo.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Ingo Molnar <mingo@...hat.com>
Subject: Re: [PATCH] RAS: Add a tracepoint for reporting memory controller
 events

On Thu, May 31, 2012 at 03:04:55PM -0300, Mauro Carvalho Chehab wrote:
> The tracepoint approach is proposed as a way to replace dmesg, so,
> dmesg is not an option.

Of course it is an option: userspace parses dmesg and gets grain value.

> Sysfs nodes for address grain won't work, as, on MCA, the grain is
> only known when an error is generated, and it is valid only together
> with an error report.

This is only true on Intel.

> Having the _same_ field exported either in sysfs or via the trace,
> depending if the grain is dynamic or if it is global-wide is a very
> crappy API, as the same information would be provided to userspace via
> different API's, with is insane.

No, what is crappy is "punishing" other drivers just because you need
grain for your drivers.

Ask Steven how expensive are 4 bytes in a tracepoint and what it means
wasting space in the ring buffer.

[ … ]

> > Again, only on those drivers which have dynamic grain. Other drivers
> > which keep outputting 'x' (and 'x' doesn't change) on every tracepoint
> > invocation don't need to output it all in the tracepoint.
> 
> So what? If only one type of granularity applies to all addresses, the
> information provided to userspace won't be wrong.

The argument is not whether it is wrong or not - it is about wasting
space unnecessarily where it counts. Write this down and hammer it to
your forehead so that you can finally understand it!

<remove some random unrelated blubber>

-- 
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Boris.

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